Answer:
3
Step-by-step explanation:
Important concept! The little number after an element represents the number of atoms in the substance. Eg. SF6. The 6 after the F tell us that there are 6 atoms of Fluorine in SF6. The coefficient or big number before a substance tells us how many of that substance there is in the equation. TO find the number of atoms in a substance with a coefficient we simply multiply the coefficient by the subscript or small number. Eg. 4SF6. The number of fluorine atoms in 4SF6 can be found my multiplying 4 and 6. 4 * 6 = 24 so there are 24 atoms of fluorine in 4SF6
Balancing the equation!
First lets list the number of atoms of each element on each side so we can see what we need to balance
SF6 + SO3 --> O2SF2
2 S 1
6 F 2
3 O 2
First lets balance the number of oxygen atoms. We can do this by adding a coefficient of 2 to SO3 and a 3 to O2SF2. After doing so lets count the number of atoms of each element again as some of them may have changed.
SF6 + 2SO3 --> 3O2SF2
3 S 3
6 F 6
6 O 6
It appears that the each element in the reaction is now balanced after adding a 2 to SO3 and a 3 to O2SF2 so the equation is now balanced.
The answer would be 3 as 3 was added to O2SF2 to balance the reaction.